DESCRIPTION (adapted from the applicant's description) The primary responsibility of the Radiological Physics Center (RPC) is to assure the National Cancer Institute (NCI) and the Clinical Trial Cooperative Groups that participating institutions have no major systematic dosimetry errors and that adequate quality assurance (QA) procedures are in place, so that participating institutions radiotherapy treatments are clinically comparable. To accomplish this goal RPC monitors the basic machine output and brachytherapy source strength, the dosimetry data used by the institution, the calculational algorithms used in treatment planning, and QA procedures. Both on-site and remote audits are undertaken. During on-site reviews, key personnel are interviewed, physical measurements are made on therapy machines, dosimetry and QA data are reviewed, treatment planning algorithms are tested and patient dose calculations are evaluated. Remote audit tools include mailed thermoluminescent dosimeters (TLD), comparison of data with RPC standard data, evaluation of calculations to verify treatment planning algorithms and review of QA procedures and records. Recently, RPC has also utilized mailed anthropomorphic phantoms to verify treatment doses for specialized techniques. RPC routinely monitors the full range of conventional as well as specialized radiation therapy techniques such as stereotaxic treatment, high dose rate brachytherapy, electron beam treatment, etc. Increasing monitoring of three dimensional (3-D) conformal treatment techniques will be a focus of this grant cycle. The RPC currently monitors 1,218 radiation therapy facilities and an average of 1,500 patient charts are annually reviewed of which approximately clinical evaluation for three Cooperative Groups (GOG, NSABP, NCCTG) and two other Groups have asked the RPC to participate in their QA operations (SWOG, RTOG) in evaluation of selected patient protocols. The RPC physicists and staff also attend Cooperative Group meetings. The RPC serves as a resource in radiation dosimetry and physics to nine Cooperative Groups and collaborates with five radiotherapy QA offices that serve these cooperative groups.